The present invention relates generally to bearings.
Foil bearings, such as disclosed in U.S. patent application Ser. Nos. 08/827,203 and 08/827,202, assigned to the assignee of the present invention, and U.S. Pat. Nos. 4,262,975; 4,277,113; 4,300,806; 4,296,976; 4,277,112; and 4,277,111 of Hooshang Heshmat (either as sole or as joint inventor), an inventor of the present invention, which applications and patents are incorporated herein by reference, include a sheet positioned to face a shaft portion for relative movement there between and means in the form of a corrugated shape having a plurality of ridges or other suitable form for resiliently supporting the sheet thereby defining a compliant hydrodynamic fluid film bearing. The bearing may be a journal bearing in which case the sheet is in surrounding relation to a shaft for relative rotational movement there between or a thrust bearing in which case the sheet bears a rotating shaft runner. Stiffness and damping are provided in a foil bearing by the smooth top foil or sheet and structural support elements which are suitably designed to provide a compliant spring support of the desired stiffness (or stiffness which is variable with load) and damping and by the hydrodynamic effects of a gas film between the shaft and the smooth top foil.
Magnetic bearings, such as disclosed in U.S. patent application Ser. No. 09/046,334, which is assigned to the assignee of the present invention, and in U.S. Pat. Nos. 5,084,643; 5,133,527; 5,202,824; and 5,666,014 of Hsaing Ming Chen (either as sole or as joint inventor), an inventor of the present invention, which applications and patents are incorporated herein by reference, include magnet means on a housing which magnetically interact with a shaft portion for adjusting the position thereof during rotation thereof. A magnetic bearing may be provided as either a journal or a thrust bearing.
Magnetic bearings may be classified as using either repulsive or attractive forces. Repulsive force systems often use permanent magnets while attractive force systems usually use electromagnets. Attraction electromagnets are usually used for magnetic suspension systems (bearings) since stiffness nearly comparable to rolling element bearings can be achieved and since active control permits variation of parameters as dictated by rotor system dynamics. An actively controlled magnetic bearing generally comprises a stator which is wound with coils to create the magnetic field and ferromagnetic laminations mounted on the rotor to interact with the stator magnetic field.
Position sensors provide feedback for control of magnetic bearings. Bias currents are conventionally applied to the electromagnets to support static loads and set up an operating flux field for linearized control. Since the flux field is equivalent to a negative spring, the bearing is inherently unstable. For reliable rotor control, both rotor position and its rate of change need to be corrected. In other words, the active magnetic bearing needs damping or velocity control, which is achieved by adding rotor velocity feedback to the current control. The rotor velocity is generally estimated from the displacement measurements through the use of a differentiator, phase-lead circuit, or state observer. In addition to dynamic stiffness and damping, basic rotor position error feedback is required to statically center the rotor. A typical magnetic bearing control is thus a gain and phase compensation network which provides a summation of (1) the time-varying position signal for dynamic stiffness control (which may be called xe2x80x9cProportionalxe2x80x9d), (2) the integral of the position signal error for static stiffness control (which may be called xe2x80x9cIntegralxe2x80x9d), and (3) the derivative of the time-varying position signal for damping (which may be called xe2x80x9cDerivativexe2x80x9d). High static stiffness is provided to keep the rotor centered in the bearing. With independent control of each of these elements, the controller, which may resultingly be called a xe2x80x9cPID controllerxe2x80x9d, allows the magnetic bearing characteristics to be varied as a function of machine operation. Lead-lag or notch filter circuits are added to the PID circuit to allow gain and phase compensation at resonant frequencies not covered by the PID circuit. Common rotordynamic controls include varying the bearing stiffness to alter lateral vibration modes, inserting damping to reduce dynamic motion, and generating rotating bearing forces to oppose or cancel rotor unbalance and harmonic forces.
The mechanical simplicity of foil bearings makes them suitable for high-speed machines such as those with cryogenic turbo-rotors with both expander and compressor wheels running at tens of thousands rpm. However, a significant effort is required to design a set of foil bearings for any new application. Furthermore, foil bearings do not lift off at low speed, thus requiring a coating on the foil for protection thereof at low speeds during start-ups and shut-downs. To make a long-lasting coating, uniform foil surface compliancy must be provided by design. Moreover, it is not easy to design an adequate amount of coulomb damping in the foil bearing crucial for rotor stability at high speeds.
Active magnetic bearings are considered to be well suited for low speed operations due to there being no metal contact, dynamic softness, and electronically maneuverable stiffness and damping. However, active magnetic bearings are vulnerable to rotor bending or structural resonances, due to non-collocation of sensors and actuators (not in same location axially as bearing center), which can easily saturate power amplifiers and make the control system unstable. Furthermore, it is difficult to provide reliable and long-lasting back-up bearings for active magnetic bearings. Conventional rolling-element type back-up bearings tend to have skidding wear and last for only a few rotor drops due to electric failures. Moreover, violent backward whirl may occur to render a rotor-bearing failure a disaster. However, some progress is being made to provide improved back-up bearings.
Since the foil bearing is considered to be advantageous for high speed operation and the magnetic bearing for low-speed operation, it is considered advantageous to combine them into a hybrid bearing having the advantages of each. The hybrid journal bearing is considered to provide, for some applications such as aircraft gas turbines, the following benefits. Since the specific capacity of a foil bearing is typically about 500 lbs. per lb. of bearing weight and since that of the active magnetic bearing is typically about 40 lbs. per lb. of bearing weight, the hybrid bearing should be much smaller, lighter in weight, and consume less power than a pure active magnetic bearing, for the same load capacity. The rotor may coast down safely on the foil bearing part in case of electric power loss to the magnetic bearing part. The foil bearing coating wear problem is no longer a problem because the magnetic bearing part can take the load at low speeds. Sub-synchronous stability can be enhanced by electronically generated damping of the magnetic bearing part. Sub-synchronous stability may be enhanced by electronically tuning the controller transfer function to obtain desirable system dynamics. The tuning of the magnetic bearing part in high frequency range may be simplified because the rotor is supported by the foil bearing part with Coulomb damping. The ability to independently vary bearing characteristics, provided by the active magnetic bearing, offers versatile rotor control. With the employment of an active magnetic bearing part, rotor speed has no direct effect of the load capacity.
It is therefore considered desirable to provide a suitably controllable hybrid journal bearing. However, there is an eccentricity of the foil bearing part, as seen in FIGS. 3 and 4, which is discussed hereinafter, which would seem to be incompatible with the lack of such eccentricity in a conventional magnetic bearing. Thus, these eccentricity differences, wherein the natural rotational center of a rotor within a magnetic bearing would be different from its natural rotational center within a foil bearing, would seem to rule out a hybrid use of both a foil bearing part and a magnetic bearing part.
If a hybrid journal bearing were to comprise magnetic control coils on opposite sides of the bearing, this would thus provide right and left magnetic planes. For a large load capacity, a foil journal bearing tends to have a length to diameter ratio close to 1. This would make the hybrid journal bearing long in the axial direction with the result that an undesirably large bending mode node could exist inside the hybrid bearing. If all four bearing coils (quadrants 1 and 3, both right and left magnetic planes, for vertical control; or quadrants 2 and 4, both right and left magnetic planes, for horizontal control) for control in one direction (vertically or horizontally) were connected to be actuated with one bi-polar power amplifier to achieve a push-pull control action, sensor non-collocation may undesirably cause the magnetic bearing actuator to produce forces which are improperly phased with respect to the shaft motion that is to be controlled.
The benefits of a hybrid foil-magnetic thrust bearing are considered to be similar to those of a hybrid foil-magnetic journal bearing. Thus, the hybrid thrust bearing for high-speed and high-temperature applications would carry more load per pound of bearing weight than a conventional thrust active magnetic bearing. The hybrid thrust bearing would have superior dynamic characteristics because the foil part, inherently a high speed bearing part, and the magnetic part, which with solid cores performs well at low frequencies, would tend to complement each other. The foil part, since it would not take up any load at start-ups and shut-downs, would not need a coating or its coating, if provided, should last a long time. Furthermore, at high speeds, the foil part would be able to take over and prevent bearing catastrophe in case of electric or control failures.
At the operating speed, the load on a thrust bearing varies from a minimum or normal thrust load to a maximum due to, for example, a compressor surge. The hybrid thrust bearing should be able to take any load amount within its capacity without xe2x80x9cthinkingxe2x80x9d (performing off-line logic calculations and making decisions in a supervising controller to re-set parameters) since there may not be enough time to do the xe2x80x9cthinkingxe2x80x9d.
It is accordingly an object of the present invention to provide a suitably controllable hybrid foil-magnetic bearing.
It is another object of the present invention to provide control of a hybrid foil-magnetic journal bearing which is substantially free of the effects of a bending mode node resulting from magnetic planes on opposite sides of the bearing and of sensor non-collocation.
In accordance with the present invention, there is provided a suitably controllable hybrid foil-magnetic journal bearing wherein the foil and magnetic bearing parts share the load at a predetermined speed. In accordance with one embodiment thereof, foil and magnetic bearing stiffnesses are continuously re-calculated and new reference eccentricities established therefrom and inputted to a controller to thereby provide control of steady state load sharing in the bearing wherein the control reference of the magnetic bearing part follows the eccentricity of the foil bearing part. In accordance with another embodiment thereof, an adaptive filter is provided to filter out vibration signals at the frequency coincident with the rotor speed from control current so that the control current need not be re-calculated at different rotor spin speeds and, thus, so that load sharing may be achieved quickly without xe2x80x9cthinking.xe2x80x9d Alternatively, the integral control coefficient may be selected such that the magnetic part static stiffness is reduced, preferably gradually, at increased rotor speed or speeds thereby allowing the rotor center to xe2x80x9cseekxe2x80x9d the foil bearing part center at the respective rotor speed so that load-sharing may be achieved automatically or without xe2x80x9cthinking.xe2x80x9d
In order to provide control of the journal bearing which is substantially free of the effects of a bending mode node resulting from magnetic planes on opposite sides of the bearing and of sensor non-collocation, in accordance with the present invention, for each axis (vertical and horizontal), a first signal is outputted, in response to input of the shaft radial position from a first sensor on a first side of the bearing housing, to a first control coil means for regulating flux on the first side of the housing, and a second signal is outputted, independently of the outputting of the first signal, in response to input of the shaft radial position from a second sensor on a second side of the bearing housing, to a second control coil means for regulating flux on the second side of the housing.
In order to provide a suitably controllable hybrid foil-magnetic thrust bearing, in accordance with the present invention, the foil bearing and magnetic bearing parts are located on opposite sides of the thrust runner for sharing the load. In accordance with one embodiment, control current is re-calculated as rotor speed changes. Alternatively, the integral gain may be selected such that, regardless of rotor speed, the control current will not need to be re-calculated at different rotor speeds so that load-sharing may be achieved automatically and thus quickly without xe2x80x9cthinking.xe2x80x9d
The above and other objects, features, and advantages of the present invention will be apparent to one of ordinary skill in the art to which the present invention pertains in the following detailed description of the preferred embodiments of the present invention when read in conjunction with the accompanying drawings wherein the same reference characters denote the same or similar parts throughout the several views.